SUMMARY Pesticides are pervasive in our environment, and have accumulated in drinking water reserves serving millions of people worldwide, causing growing concern since the potential adverse health effects are not fully understood. In 2017 alone, the world used about 6.8 million tons of pesticides, of which the world's largest usage is in China and the United States. More recently it has been discovered that pesticides have strong effects on microbial communities, extinguishing entire clades while promoting growth in others. Shifts in microbial composition have been associated with brain development, mood and behavior suggesting a strong link between the gut and the brain. We hypothesize that there may exist an unappreciated microbiome component to the adverse health effects of pesticide exposure. In this proposal, we will use a comprehensive approach to identify the microbial components of pesticide susceptibility to neurodevelopmental delay. Specifically, in Aim 1 we will investigate the impact of prenatal exposure to 11 targeted pesticides on child neurodevelopment in a large well-defined human Mother and Child Microbiome Cohort (MCMC) consisting of 3000 mother and child pairs. Study participants are currently followed up regularly until the children reach 4 years of age. Comprehensive artificial intelligence analysis of pesticide exposure and children's early neurodevelopment data will identify key pesticides that negatively impact child development. In Aim 2, we hypothesize that prenatal exposure to pesticides can shift the composition of the GM and subsequently lead to abnormal neurodevelopment. We will integrate the child gut microbiome data with maternal pesticide exposure during pregnancy and child neurodevelopment data to investigate the impact of prenatal exposure to targeted pesticides on child gut microbiome and development. This Aim will identify specific gut bacteria or clades associated with child neurodevelopment. In Aim 3, we will use conventional and germ-free mouse models to elucidate mechanisms by which pesticide exposure can affect the gut microbiome and behavior. Conventionally housed pregnant mice will be exposed to pesticides and the gut microbiome and behavior (anxiety, memory, motor performance, social interaction) will be analyzed in their offspring. Also, germ-free mice will be colonized with specific bacteria or bacterial clades to study the causality of the impact of the GM on behavior. The long-term implications for human health are significant ? it may be possible to increase tolerance/robustness to pesticide exposure by providing sensitive individuals with probiotics that have select degradation pathways. Moreover, this understanding is important in order to formulate novel preventive strategies including educating health practitioners, families, and public health providers and organizations regarding the potential risks of environmental exposures.